ANHB1101 Human Biology I: Becoming Human - Hormonal Control of Reproduction

Hormonal Control of Reproduction

Learning Objectives

  • List the main groups of reproductive hormones and give examples of each.
  • Explain the function, control, and action of GnRH, gonadotrophins, estradiol, progesterone, testosterone, and inhibin.
  • Explain positive and negative feedback.
  • Describe the hormonal control of the testicular and ovarian axis.
  • Apply the above to explain the action of hormonal contraceptives and the effects of anabolic steroids.

Communication

  • Hormones are reproductive messengers.
  • Examples:
    • Breast
    • SV = seminal vesicle
    • VD = vas deferens
    • Oviduct
    • Body hair
    • Uterus
    • Breast
    • Ovary
    • SV
    • VD
    • Vagina
    • Testis
    • Penis

Hormone Action - Revision

  • Hormones are chemical messengers (hormoneo, ‘I excite’).
  • Produced by endocrine glands/cells.
  • Act on target cells/tissues.
  • Target cells express specific receptors (membrane-bound or intracellular).

Chemical Structures of Reproductive Hormones

  • Peptides (GnRH, LH, FSH, hCG, inhibin, oxytocin, prolactin).
  • Steroids (testosterone, estradiol, progesterone, cortisol).
  • Eicosanoids (prostaglandins).

Peptide Hormone Action

  • Peptide hormones bind to membrane-bound receptors and require action of second messengers.
  • Ligand binds to membrane-bound receptor.
  • G protein is activated using GTP and GDP.
  • Adenylyl cyclase is activated.

Peptide Hormone Action Requires Second Messengers

  • Examples: GnRH, LH, and FSH.
  • Ligand binds to membrane-bound receptor.
  • Cell response is triggered.
  • GTP and GDP are used in the process.
  • Adenylyl cyclase becomes active.
  • ATP is converted to cAMP.

Steroid Receptors

  • Steroid hormones enter the cytoplasm and bind to receptors.
  • Molecular chaperones are involved.
  • Receptor-hormone complex is formed.
  • Phosphorylation occurs.
  • Acceptor-Chromatin interaction.
  • Transcription leads to mRNA production.
  • Translation occurs in the ribosome, creating new protein.
  • Examples: estradiol, progesterone, testosterone.

Types of Hormone Action

  • Endocrine: Circulating hormone affects distant target cells.
  • Paracrine: Hormone affects nearby target cells.
  • Autocrine: Hormone affects the same cell.
  • Exocrine

Control of the Endocrine System

  • Negative Feedback
  • Positive Feedback

Negative Feedback

  • Non-biological example: Room temperature control.
  • Room temperature falls to 66°F(19°C)66°F (19°C).
  • Thermostat activates furnace.
  • Heat output increases.
  • Room temperature rises to 70°F(21°C)70°F (21°C).
  • Thermostat shuts off the furnace.
  • Room cools down.
  • Furnace turned off at 70°F70°F. Set point is 68°F68°F. Furnace turned on at 66°F66°F.

Negative Feedback (General)

  • Gland releases a hormone.
  • Hormone acts on a target tissue.
  • Response from target tissue inhibits further hormone release.
  • Promotes homeostasis.

Example: LH / Testosterone Negative Feedback

  • Anterior pituitary releases LH.
  • LH acts on Leydig cells.
  • Testosterone production is stimulated.
  • Testosterone inhibits LH release from the anterior pituitary.

Positive Feedback

  • Gland releases a hormone.
  • Hormone acts on a target tissue.
  • Response from target tissue stimulates further hormone release.
  • Highly unstable.

Example: Positive Feedback - Childbirth

  • Posterior pituitary releases oxytocin.
  • Oxytocin causes powerful contractions of the myometrium of the uterus.
  • Nervous signal further stimulates oxytocin release until birth.

Hierarchy of Control

  • Environmental Cues.
  • Hypothalamus releases Gonadotrophin-releasing hormone (GnRH).
  • Pituitary releases Gonadotrophins (LH & FSH).
  • Gonads (ovary and testis) produce Estradiol, Progesterone, Testosterone, and Inhibin.

Female & Male Reproductive Tract

  • Males produce gametes and transport gametes for fertilization.
  • Females produce gametes, transport gametes for fertilization, provide a receptacle for fertilization, embryogenesis, and fetal development, give birth, and nurture the newborn (lactation and care).

Differences in Male vs Female Function Reflected in Feedback Mechanisms

  • Female fertility is episodic: cycle of ovarian activity.
  • Male gamete and steroid production is continuous from puberty.
  • Difference reflected in feedback mechanisms of gonadal steroids on gonadotrophins.
    • Male: -ve feedback.
    • Female: -ve AND +ve feedback.

Endocrine Control of Spermatogenesis

  • Hypothalamus releases GnRH.
  • Anterior pituitary releases LH and FSH.
  • LH acts on Leydig cells to produce testosterone (+ve feedback).
  • FSH acts on Sertoli cells.
  • Testosterone and Inhibin provide -ve feedback to the hypothalamus and anterior pituitary.

Cyclic Changes

  • Gonadotrophins (LH & FSH) change cyclically.
  • Ovarian Steroids (Estrogen & Progesterone) change cyclically.
  • Uterine Endometrium goes through proliferative and secretory phases.
  • Follicular phase and Luteal phase occur in the ovaries.

Endocrine Control of Ovarian Cycle

  • Hypothalamus releases GnRH (+ve feedback).
  • Anterior pituitary releases FSH (-ve feedback).
  • Late luteal phase of the previous cycle leads to the early follicular phase of the current cycle.
  • Inhibin is involved.
  • Follicles are stimulated by FSH.

Endocrine Control of Ovarian Cycle (cont.)

  • Hypothalamus releases GnRH (+ve feedback).
  • Anterior pituitary releases LH and FSH (-ve feedback).
  • Early follicular phase of the current cycle.
  • Inhibin is involved.
  • Follicles respond to FSH.
  • Estradiol is produced.

Endocrine Control of Ovarian Cycle (cont.)

  • Hypothalamus releases GnRH (+ve feedback).
  • Anterior pituitary releases LH and FSH (-ve feedback).
  • Late follicular phase – just prior to ovulation.
  • Inhibin is involved.
  • FSH stimulates the dominant follicle.
  • Estradiol is produced in high amounts (+++).

Endocrine Control of Ovarian Cycle (cont.)

  • Hypothalamus releases GnRH (+ve feedback).
  • Anterior pituitary releases LH and FSH (-ve feedback).
  • Late follicular phase – ovulation.
  • Inhibin is involved.
  • FSH stimulates the dominant follicle.
  • Estradiol is produced in very high amounts (+++).
  • LH surge occurs (+++).

Ovulation

  • LH surge leads to ovulation (+ve feedback loop is broken).
  • Formation of corpus luteum from the ruptured follicle.

Endocrine Control of Ovarian Cycle (cont.)

  • Hypothalamus releases GnRH (+ve feedback).
  • Anterior pituitary releases LH and FSH (-ve feedback).
  • Luteal phase.
  • Inhibin is involved.
  • Estradiol and Progesterone are produced by the corpus luteum.
  • Corpus luteum has a finite lifespan of 10-12 days.

Endocrine Control of Ovarian Cycle (cont.)

  • Hypothalamus releases GnRH (+ve feedback).
  • Anterior pituitary releases LH and FSH (-ve feedback).
  • Luteal phase.
  • Inhibin levels decrease (↓).
  • Estradiol levels decrease (↓).
  • Progesterone levels decrease (↓).
  • Corpus luteum dies.
  • Follicular growth is re-initiated.

Pregnancy

  • ++ estradiol (corpus luteum).
  • ++ progesterone (corpus luteum).
  • Embryo implants in the uterus.
  • Hormone signal to CL (Corpus Luteum).
  • Pregnancy: GnRH, FSH, LH are suppressed (-ve feedback).

Combined Oral Contraceptive Pill

  • Contains synthetic estrogen + progesterone.
  • Suppresses FSH and GnRH (-ve feedback).
  • Prevents follicular development beyond the primordial follicle stage.
  • Prevents formation of Corpus luteum and Corpus albicans

Combined Oral Contraceptive Pill (cont.)

  • Continuous administration of synthetic estrogen and progesterone (21 days).
  • Suppresses follicular growth.
  • Placebo pills (7 days) cause a withdrawal bleed.
  • Relaxation of -ve feedback, ↑ gonadotrophin, ↑ follicular growth.
  • Implications for method failure.